Provides enahncement fguillot/miniflux #162 "Provide original link to any audio/video enclosures".

Essentially, if an enclosure tag exists in the item, then the item menu will contain
a link 'multimedia [speaker symbol]' to the media url. Otherwise, if there is no
enclosure tag, there is no menu item shown.

This feature required adding an enclosure column to the items table (models/schema.php)
and bumping the DB version to 21 (models/config.php). I also added enclosure tag logic
to the Rss20 parser, the model/item.php and template/item.php files.